I know this isn't what you asked, but I would think long and bloody hard about getting back together with your ex.
From what you've said, it doesn't sound like a good idea at all.
FWIW, I'm a divorced parent and my child's married home was a hell of a lot broken than her two post divorced homes.
All you're doing (financially) in getting back with your ex is adding his debts to your family financial situation and reducing your benefits.
It sounds like you had good reason to ditch him on previous occasions.
Have you applied back to McD as you have experience there? More chance of fitting it round his shifts, I presume?
I would stay single, claim everything you can that way, make sure you are getting maintenance if appropriate, and find a job even if it is 4 hours a week just so you start to have a recent employment history.
Even if he works variable shifts, he's a father - he needs thas step up. Minimum, he needs to have the kids and never work "x" set hours so that you can work that time to build your employment history.